The present invention relates to inhalation apparatus and in particular to medical humidifiers.
Medical humidifiers are used with intensive care ventilators to heat and humidify dry medical gases to specified levels before said gases are inhaled by a patient.
A medical humidifier is known from UK patent No. 1448473 which includes a reservoir for water in communication with a humidification chamber. The humidification chamber has an inlet for gas to be humidified and an outlet for warm humidified gas. Heat is transferred to the humidification chamber by a heating coil. A delivery line extends between the outlet port of the humidification chamber and a delivery point.
In a modification of this known medical humidifier, a control system including a temperature sensor controls the heater power to the humidifying chamber to maintain the outlet gas temperature at a set value despite changes in gas flowrate through the humidification chamber. The design of the chamber ensures the outlet gas is fully saturated with water vapour by including a large water evaporating surface using wick materials.
However, there is a problem associated with this known type of humidifier in that the temperature sensor is thermally isolated from the heater and relies on gas flow to indicate the gas temperature in the humidification chamber. If there is a period in which no gas flows, then the temperature of the gas at the sensor starts to fall and the control system responds to this fall by increasing the power to the heater and this results in a system heading towards its maximum operating temperature. When the gas flow is turned on again, a transient temperature overshoot occurs.